The process of soldering is one of the most widely used joining techniques. Even wider use of soldering for joining workpieces would be possible absent several limitations inherent in conventional solder processes. Typically, conventional solder processes can be used successfully only (1) if the surfaces of the workpieces to be joined are cleaned and free of oxides before application of the solder, to free the surfaces of any oxide layers present and ensure good contact between the solder and the workpiece surfaces; (2) if a precoating with a flux is used at the same time as the solder; or (3) if the workpiece surfaces are cleaned and a precoating with a flux is used. These limitations mean that the workpiece surfaces to be soldered require a complicated pretreatment, that the soldering operation is made more complex by the use of added flux, or both. In addition, the risk exists that, after the soldering process, flux residues will remain on the soldered workpiece surfaces. Residual flux may cause problems in further processing steps or impair the long-term durability of the soldered joints. Finally, some soldering processes are hazardous to health, the environment, or both.
Some commercial soldering processes use soft solder alloys, which comprise tin and/or lead and possibly silver, and have a process temperature of about 200.degree. C. These processes have the further limitation that they wet many materials either not at all or only very poorly. Therefore, these processes cannot be used to join workpieces comprised of poorly wettable or entirely nonwettable materials such as ceramics.
Some soldering processes attempt to overcome this limitation by using "activated" soft solders comprising an admixture of the soft solders with titanium (an "active" metal). These processes have significantly improved wetting characteristics. A serious limitation of these processes is, however, that they require process temperatures between 600.degree. C. to 900.degree. C. and require a high vacuum or a pure shielding gas. The high processing temperature severely limits the choice of solderable materials. Furthermore, the need for a vacuum or shielding gas complicates the soldering operation and, in many cases, precludes application of the process at all. For certain special cases, the process of eutectic copper bonding can be used; this process is even more expensive and complicated.
Therefore, an object of the present invention is to provide a soldering process, for joining workpieces, which enables more versatile applicability of the soft-solder technique. More specific objects of the present invention are to provide a soldering process that functions even in oxygen-containing atmospheres such as, for example, in air; has a relatively low processing temperature; and wets even poorly wettable surfaces. Still another object of the present invention is to provide a process that avoids the need for a flux.